In recent years, a power supply system composed by combining, for example, a power generator or charger utilizing natural energy such as a solar battery and a wind power generator, a storage battery, and a control system for these has been used as a self-contained power supply or a power leveling mechanism thereof. Such a power supply system is used as a single independent power supply and is sometimes operated while inputting and outputting power to/from the ordinary commercial power source in a so-called grid connection. Recently, like the so-called smart grid, the efficiency of the overall power supply is improved by the reverse power flow of the generated power to the grid in consideration of the power usage conditions in the grid and by storing the surplus power in a storage battery when the grid load is decreased and the capacity of the storage battery is decreased.
However, in the case of utilizing natural energy, the energy source is natural phenomena such as sunshine and wind conditions, and the human cannot take complete control thereof. For example, in the case of the solar power generation, power generated by the solar battery is supplied to a load while charging the storage battery with surplus power in the daytime during which sunshine is abundant, and necessary power is supplied to the load by the discharging from the storage battery in the daytime during which sunshine is scarce or the nighttime.
However, in the case of the solar battery, for example, when the sunshine in the daytime is abundant and the storage battery is kept supplied with surplus power, the storage battery may be overcharged, and it accelerates the degradation of the storage battery. Meanwhile, when sunshine is scarce, since the storage battery has to keep supplying power to the load, the storage battery may be overdischarged, and it also accelerates the degradation of the storage battery. Although the wind power generation has an advantage of receiving no direct effect from the amount of sunshine, it is difficult to maintain a constant wind velocity, and the wind power generation has characteristics that wind velocity, wind volume, and wind direction are varied by the minute.
Therefore, in order to stably operate the power supply system and power generation facility, a storage battery with a large capacity is necessary for the former case of the solar battery and a capacitor and storage battery having a large electric double layer are necessary for the latter case of the wind power generation. In addition, a storage battery is necessary also for the stable cooperation with the electrical grid, and the partially charged state referred to as PSOC (partial state of charge) of the storage battery has to be controlled within a given range.
For this reason, it is preferable to use the storage battery while constantly keeping the battery capacity within the range in which normal charging/discharging is possible. For example, Japanese Patent Application Laid-Open Publication No. 2010-63359 (Patent Document 1) describes a power supply system including a solar battery, a lead storage battery, a charge control device which charges the lead storage battery with power output from the solar battery, and a photosensor. The charge control device has a function of opening a charge circuit when the voltage of the storage battery rises to a first set voltage during the charging and closing the charge circuit to recharge the storage battery when the voltage of the storage battery drops to a second set voltage lower than the first set voltage. Also, when a charge current to the lead storage battery is equal to or smaller than a set current, the charge control device does not open the charge circuit and continues the charging even if the voltage of the storage battery rises to a voltage equal to or higher than the first set voltage, and when an illuminance value measured by the photosensor drops to a value equal to or smaller than a set value, the charge control device controls the lead storage battery to supply power to a load. In this manner, decrease in the storage battery capacity caused by insufficient charging or overdischarging of the storage battery can be suppressed.
Also, for example, Japanese Patent Application Laid-Open Publication No. 2008-97941 (Patent Document 2) describes a technique for a charge management system which manages the charging of a storage battery. The charge management system includes a discharge capacity acquiring unit which acquires a discharge capacity of the storage battery and a charging unit which charges the storage battery at a charge capacity in accordance with the discharge capacity acquired by the discharge capacity acquiring unit, and by summing up the charge and discharge currents, the storage battery can be charged at a charge capacity in accordance with the discharge capacity up to the point of time of charging.
Furthermore, for example, Japanese Patent Application Laid-Open Publication No. 2012-37464 (Patent Document 3) describes a technique for a lead storage battery and a lead storage battery system which can be used for large-scale applications including wind power generation. The lead storage battery system includes: a battery state measuring unit; an SOC (State of Charge) model; an SOC estimating unit which estimates an SOC based on measurement information obtained by the battery state measuring unit and SOC model information; an SOC transition DB in which a state of transition of the SOC of the lead storage battery is recorded; an SOC transition history managing unit which records an estimated SOC value in the SOC transition DB to check a state of transition of the SOC; a lead storage battery degradation model; a uniform charging optimum planning unit which plans a method of carrying out optimum charging of the lead storage battery based on the SOC transition state from the SOC transition history managing unit and information of the deterioration model; an SOC transition information/uniform charging information output unit; and a uniform charging control unit which carries out uniform charging of the lead storage battery in accordance with the plan determined by the uniform charging optimum planning unit, and by changing the interval of executing uniform charging of the lead storage battery in accordance with the usage conditions of the lead storage battery (transition of SOC), the service life of the lead storage battery is extended, and the frequency of uniform charging carried out merely for grasping the SOC is reduced so as to achieve a cost advantage.